1. Field of the Invention
The present invention relates to a vector for treating tumors by gene therapy, in particular in connection with a radiotherapy, whose DNA sequence has at least one tissue-specific promoter and at least one therapeutic gene whose expression is controlled by the promoter.
2. Related Prior Art
DE 44 44 949 C1 discloses a vector of this type.
For the purposes of the present invention, “tumors” means both malignant and benign tumors.
Malignant neoplastic disorders account for approx. 30% of deaths in the civilized world, and there is at present no safe therapy for any tumor yet, in spite of worldwide efforts over the last decades. Many tumors can be treated only with difficulties, if at all.
Examples of benign tumors include tumors of the vascular wall for which partly the same therapies are used as for the treatment of malignant tumors, i.e. cancer. Such tumors of the vascular wall form as recurring stenoses essentially due to induction of smooth muscle-cell proliferation in the vascular wall caused by “balloon dilatations” (PTCA) for the therapy of local stenoses of the vascular wall which may limit an organ's blood supply. The treatment of such arteriosclerotic disorders includes in addition to balloon dilatation also bypass surgery, stents and other alternative therapeutic methods which, however, likewise have the problem of recurring stenosis, i.e. constriction of the lumen of the treated vessel due to a benign tumor.
Besides surgical removal of both malignant and benign tumors and the treatment thereof with cytostatics, radiotherapy represents from the present point of view one of the most important pillars of tumor therapy. In this connection, the probability of destruction of the tumor depends on the dose administered, with the dose to be administered being limited by the radiosensitivity of normal tissue which inevitably is also irradiated. Therapeutic successes thus depend on the relative radiosensitivity of the tumor cells compared with the cells of the neighboring tissue.
Consequently, an increase in the therapeutic range due to selective radiosensitization of tumor cells would mean a significant step forward in the treatment of tumors and improve the rates of cure. For this reason, pharmacological radiosensitizers which ought to make tumor cells more sensitive to radiation are already in use clinically.
Gene therapy, for the first time, offers the opportunity of achieving significant progress in controlling cancer by making it possible to use therapeutic genes for enhancing radiotherapy or therapy with cytostatics.
In this connection, viral vector systems play a great part in transducing therapeutic genes. Besides retroviral vector systems which, to a certain extent, prefer proliferating cells and very often integrate into the cellular genome, especially adenoviral vector systems which make it possible to attain a high titre of virus particles and which have good transduction efficiency and a very low rate of integration are in discussion; see Ali et al., Gene Ther. (1991), Volume 1, 367-384.
It is crucial for a reliable gene therapy to use the therapeutic gene only in the desired target cells. It would therefore be desirable to use in the gene therapy of tumors a tumor cell-specific promoter which is active in various tumor species and not active in all types of normal tissue. Such a promoter, however, has not yet been found.
In this connection, the initially mentioned DE 44 44 949 C1 describes a vector for the gene therapy of patients who, after surgical removal of a tumor, undergo an aftertreatment using conventional radiation and/or cytostatic methods. The vector contains an expressible DNA insert which is located behind a promoter active in tumor cells and codes for the DNA-binding domain (DBD) of a poly(ADP-ribose) polymerase (PARP). The idea on which that publication is based is to inhibit the activity of the enzyme PARP which is required for repairing damaged DNA by adding DBD molecules so that repair of damaged DNA is prevented.
As example of a tissue-specific promoter the MVM P4 promoter is mentioned. The vector may be a viral vector, and the viruses are replication-incompetent and can be complemented in trans in order to obtain viruses which code for DBD but are unable to propagate in patients.
It has turned out to be a disadvantage of the known vector that the P4 promoter does not yet have the tissue specificity required for a safe gene therapy so that DNA repair is also inhibited in normal tissue, and this is, for reasons that need no further explanation, undesirable.
Most gene therapies therefore include an enhancement of the tumor-specific immune response, i.e. they are a priori systemic, since the local increase in the immune response is not limited to the tumor. At the same time, this is also a desired advantage, because in principle it is also possible to destroy metastases using this therapy.
In order to allow a more or less local gene therapy, the use of retroviruses which transduce genes only into dividing cells has already been discussed. Furthermore, adenovirus mutants were proposed, which ought to replicate only in p53-negative tumor cells; Heise et al., Nat. Med. (1997), Volume 3, 639-645. It is assumed here that all p53-positive cells can prevent proliferation of the adenovirus.
In addition, Joki et al., Hum. Gene Ther. (1995), Volume 6, 1507-1513 have already proposed using radiation-inducible promoters and Nettelbeck et al., Adv. Exp. Med. Biol. (1998), Volume 451, 437-440 proposed using cell cycle-specific promoters.
However, all of these known solutions have specific disadvantages. The concept of immunotherapy has the principal disadvantage that immunosuppressed patients cannot respond to it, the neoplastic disorder itself often additionally adversely affecting the immune system of the said patients. Furthermore, it has turned out that an immunotherapeutic gene or a “suicide gene” which kills the target cell is on its own not sufficient for in vivo administrations; Uckert et al., HUM. GENE THER. (1998), Volume 9, 855-865. The combination of several immunostimulatory genes, where appropriate in connection with suicide genes, has also up to now not led to a breakthrough which would allow a standard application for a given neoplastic disorder.
The abovementioned retroviruses are furthermore not exclusively selective for tumors but infect all dividing cell types, as long as an appropriate receptor is present. Moreover, infection of a healthy cell carries the risk of insertion mutagenesis, since retroviruses integrate into the cellular genome. Replication-competent adenoviruses which have been proposed for a specific tumor therapy are also not specific for tumor cells, as has been proved since then, the effect also being independent of the p53 state; Rothmann et al., J. Virol. (1998), Volume 72, 9470-9478.
The previously described use of suicide genes, too, has disadvantages, since the said suicide genes increase the side effects in healthy proliferating tissues. This problem could be avoided only if the suicide genes were expressed tumor-specifically.